/*
 * jchuff.c
 *
 * Copyright (C) 1991-1995, Thomas G. Lane.
 * This file is part of the Independent JPEG Group's software.
 * For conditions of distribution and use, see the accompanying README file.
 *
 * This file contains Huffman entropy encoding routines.
 *
 * Much of the complexity here has to do with supporting output suspension.
 * If the data destination module demands suspension, we want to be able to
 * back up to the start of the current MCU.  To do this, we copy state
 * variables into local working storage, and update them back to the
 * permanent JPEG objects only upon successful completion of an MCU.
 */

#define JPEG_INTERNALS
#include "jinclude.h"
#include "jpeglib.h"
#include "jchuff.h"      /* Declarations shared with jcphuff.c */


/* Expanded entropy encoder object for Huffman encoding.
 *
 * The savable_state subrecord contains fields that change within an MCU,
 * but must not be updated permanently until we complete the MCU.
 */

typedef struct {
    INT32 put_buffer;   /* current bit-accumulation buffer */
    int   put_bits;     /* # of bits now in it */
    int   last_dc_val[MAX_COMPS_IN_SCAN]; /* last DC coef for each component */
} savable_state;

/* This macro is to work around compilers with missing or broken
 * structure assignment.  You'll need to fix this code if you have
 * such a compiler and you change MAX_COMPS_IN_SCAN.
 */

#ifndef NO_STRUCT_ASSIGN
#define ASSIGN_STATE( dest, src )  ( ( dest ) = ( src ) )
#else
#if MAX_COMPS_IN_SCAN == 4
#define ASSIGN_STATE( dest, src )  \
    ( ( dest ).put_buffer = ( src ).put_buffer, \
     ( dest ).put_bits = ( src ).put_bits, \
     ( dest ).last_dc_val[0] = ( src ).last_dc_val[0], \
     ( dest ).last_dc_val[1] = ( src ).last_dc_val[1], \
     ( dest ).last_dc_val[2] = ( src ).last_dc_val[2], \
     ( dest ).last_dc_val[3] = ( src ).last_dc_val[3] )
#endif
#endif


typedef struct {
    struct jpeg_entropy_encoder pub;/* public fields */

    savable_state saved;    /* Bit buffer & DC state at start of MCU */

    /* These fields are NOT loaded into local working state. */
    unsigned int restarts_to_go;/* MCUs left in this restart interval */
    int          next_restart_num; /* next restart number to write (0-7) */

    /* Pointers to derived tables (these workspaces have image lifespan) */
    c_derived_tbl * dc_derived_tbls[NUM_HUFF_TBLS];
    c_derived_tbl * ac_derived_tbls[NUM_HUFF_TBLS];

#ifdef ENTROPY_OPT_SUPPORTED    /* Statistics tables for optimization */
    long * dc_count_ptrs[NUM_HUFF_TBLS];
    long * ac_count_ptrs[NUM_HUFF_TBLS];
#endif
} huff_entropy_encoder;

typedef huff_entropy_encoder * huff_entropy_ptr;

/* Working state while writing an MCU.
 * This struct contains all the fields that are needed by subroutines.
 */

typedef struct {
    JOCTET *       next_output_byte; /* => next byte to write in buffer */
    size_t         free_in_buffer; /* # of byte spaces remaining in buffer */
    savable_state  cur;     /* Current bit buffer & DC state */
    j_compress_ptr cinfo;   /* dump_buffer needs access to this */
} working_state;


/* Forward declarations */
METHODDEF boolean encode_mcu_huff JPP( ( j_compress_ptr cinfo,
                                         JBLOCKROW * MCU_data ) );
METHODDEF void finish_pass_huff JPP( (j_compress_ptr cinfo) );
#ifdef ENTROPY_OPT_SUPPORTED
METHODDEF boolean encode_mcu_gather JPP( ( j_compress_ptr cinfo,
                                           JBLOCKROW * MCU_data ) );
METHODDEF void finish_pass_gather JPP( (j_compress_ptr cinfo) );
#endif


/*
 * Initialize for a Huffman-compressed scan.
 * If gather_statistics is TRUE, we do not output anything during the scan,
 * just count the Huffman symbols used and generate Huffman code tables.
 */

METHODDEF void
start_pass_huff( j_compress_ptr cinfo, boolean gather_statistics ) {
    huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
    int ci, dctbl, actbl;
    jpeg_component_info * compptr;

    if ( gather_statistics ) {
#ifdef ENTROPY_OPT_SUPPORTED
        entropy->pub.encode_mcu = encode_mcu_gather;
        entropy->pub.finish_pass = finish_pass_gather;
#else
        ERREXIT( cinfo, JERR_NOT_COMPILED );
#endif
    } else {
        entropy->pub.encode_mcu = encode_mcu_huff;
        entropy->pub.finish_pass = finish_pass_huff;
    }

    for ( ci = 0; ci < cinfo->comps_in_scan; ci++ ) {
        compptr = cinfo->cur_comp_info[ci];
        dctbl = compptr->dc_tbl_no;
        actbl = compptr->ac_tbl_no;
        /* Make sure requested tables are present */
        /* (In gather mode, tables need not be allocated yet) */
        if ( ( dctbl < 0 ) || ( dctbl >= NUM_HUFF_TBLS ) ||
            ( ( cinfo->dc_huff_tbl_ptrs[dctbl] == NULL ) && ( !gather_statistics ) ) ) {
            ERREXIT1( cinfo, JERR_NO_HUFF_TABLE, dctbl );
        }
        if ( ( actbl < 0 ) || ( actbl >= NUM_HUFF_TBLS ) ||
            ( ( cinfo->ac_huff_tbl_ptrs[actbl] == NULL ) && ( !gather_statistics ) ) ) {
            ERREXIT1( cinfo, JERR_NO_HUFF_TABLE, actbl );
        }
        if ( gather_statistics ) {
#ifdef ENTROPY_OPT_SUPPORTED
            /* Allocate and zero the statistics tables */
            /* Note that jpeg_gen_optimal_table expects 257 entries in each table! */
            if ( entropy->dc_count_ptrs[dctbl] == NULL ) {
                entropy->dc_count_ptrs[dctbl] = (long *)
                                                ( *cinfo->mem->alloc_small )( (j_common_ptr) cinfo, JPOOL_IMAGE,
                                                                             257 * SIZEOF( long ) );
            }
            MEMZERO( entropy->dc_count_ptrs[dctbl], 257 * SIZEOF( long ) );
            if ( entropy->ac_count_ptrs[actbl] == NULL ) {
                entropy->ac_count_ptrs[actbl] = (long *)
                                                ( *cinfo->mem->alloc_small )( (j_common_ptr) cinfo, JPOOL_IMAGE,
                                                                             257 * SIZEOF( long ) );
            }
            MEMZERO( entropy->ac_count_ptrs[actbl], 257 * SIZEOF( long ) );
#endif
        } else {
            /* Compute derived values for Huffman tables */
            /* We may do this more than once for a table, but it's not expensive */
            jpeg_make_c_derived_tbl( cinfo, cinfo->dc_huff_tbl_ptrs[dctbl],
                                     &entropy->dc_derived_tbls[dctbl] );
            jpeg_make_c_derived_tbl( cinfo, cinfo->ac_huff_tbl_ptrs[actbl],
                                     &entropy->ac_derived_tbls[actbl] );
        }
        /* Initialize DC predictions to 0 */
        entropy->saved.last_dc_val[ci] = 0;
    }

    /* Initialize bit buffer to empty */
    entropy->saved.put_buffer = 0;
    entropy->saved.put_bits = 0;

    /* Initialize restart stuff */
    entropy->restarts_to_go = cinfo->restart_interval;
    entropy->next_restart_num = 0;
}


/*
 * Compute the derived values for a Huffman table.
 * Note this is also used by jcphuff.c.
 */

GLOBAL void
jpeg_make_c_derived_tbl( j_compress_ptr cinfo, JHUFF_TBL * htbl,
                         c_derived_tbl ** pdtbl ) {
    c_derived_tbl * dtbl;
    int p, i, l, lastp, si;
    char huffsize[257];
    unsigned int huffcode[257];
    unsigned int code;

    /* Allocate a workspace if we haven't already done so. */
    if ( *pdtbl == NULL ) {
        *pdtbl = (c_derived_tbl *)
                 ( *cinfo->mem->alloc_small )( (j_common_ptr) cinfo, JPOOL_IMAGE,
                                              SIZEOF( c_derived_tbl ) );
    }
    dtbl = *pdtbl;

    /* Figure C.1: make table of Huffman code length for each symbol */
    /* Note that this is in code-length order. */

    p = 0;
    for ( l = 1; l <= 16; l++ ) {
        for ( i = 1; i <= (int) htbl->bits[l]; i++ ) {
            huffsize[p++] = (char) l;
        }
    }
    huffsize[p] = 0;
    lastp = p;

    /* Figure C.2: generate the codes themselves */
    /* Note that this is in code-length order. */

    code = 0;
    si = huffsize[0];
    p = 0;
    while ( huffsize[p] ) {
        while ( ( (int) huffsize[p] ) == si ) {
            huffcode[p++] = code;
            code++;
        }
        code <<= 1;
        si++;
    }

    /* Figure C.3: generate encoding tables */
    /* These are code and size indexed by symbol value */

    /* Set any codeless symbols to have code length 0;
     * this allows emit_bits to detect any attempt to emit such symbols.
     */
    MEMZERO( dtbl->ehufsi, SIZEOF( dtbl->ehufsi ) );

    for ( p = 0; p < lastp; p++ ) {
        dtbl->ehufco[htbl->huffval[p]] = huffcode[p];
        dtbl->ehufsi[htbl->huffval[p]] = huffsize[p];
    }
}


/* Outputting bytes to the file */

/* Emit a byte, taking 'action' if must suspend. */
#define emit_byte( state, val, action )  \
    { *( state )->next_output_byte++ = (JOCTET) ( val );  \
      if ( -- ( state )->free_in_buffer == 0 ) { \
          if ( !dump_buffer( state ) )  \
          { action; } } }


LOCAL boolean
dump_buffer( working_state * state ) {
/* Empty the output buffer; return TRUE if successful, FALSE if must suspend */
    struct jpeg_destination_mgr * dest = state->cinfo->dest;

    if ( !( *dest->empty_output_buffer )( state->cinfo ) ) {
        return FALSE;
    }
    /* After a successful buffer dump, must reset buffer pointers */
    state->next_output_byte = dest->next_output_byte;
    state->free_in_buffer = dest->free_in_buffer;
    return TRUE;
}


/* Outputting bits to the file */

/* Only the right 24 bits of put_buffer are used; the valid bits are
 * left-justified in this part.  At most 16 bits can be passed to emit_bits
 * in one call, and we never retain more than 7 bits in put_buffer
 * between calls, so 24 bits are sufficient.
 */

INLINE
LOCAL boolean
emit_bits( working_state * state, unsigned int code, int size ) {
/* Emit some bits; return TRUE if successful, FALSE if must suspend */
/* This routine is heavily used, so it's worth coding tightly. */
    register INT32 put_buffer = (INT32) code;
    register int put_bits = state->cur.put_bits;

    /* if size is 0, caller used an invalid Huffman table entry */
    if ( size == 0 ) {
        ERREXIT( state->cinfo, JERR_HUFF_MISSING_CODE );
    }

    put_buffer &= ( ( (INT32) 1 ) << size ) - 1;/* mask off any extra bits in code */

    put_bits += size;   /* new number of bits in buffer */

    put_buffer <<= 24 - put_bits;/* align incoming bits */

    put_buffer |= state->cur.put_buffer;/* and merge with old buffer contents */

    while ( put_bits >= 8 ) {
        int c = (int) ( ( put_buffer >> 16 ) & 0xFF );

        emit_byte( state, c, return FALSE );
        if ( c == 0xFF ) {  /* need to stuff a zero byte? */
            emit_byte( state, 0, return FALSE );
        }
        put_buffer <<= 8;
        put_bits -= 8;
    }

    state->cur.put_buffer = put_buffer;/* update state variables */
    state->cur.put_bits = put_bits;

    return TRUE;
}


LOCAL boolean
flush_bits( working_state * state ) {
    if ( !emit_bits( state, 0x7F, 7 ) ) {/* fill any partial byte with ones */
        return FALSE;
    }
    state->cur.put_buffer = 0;  /* and reset bit-buffer to empty */
    state->cur.put_bits = 0;
    return TRUE;
}


/* Encode a single block's worth of coefficients */

LOCAL boolean
encode_one_block( working_state * state, JCOEFPTR block, int last_dc_val,
                  c_derived_tbl * dctbl, c_derived_tbl * actbl ) {
    register int temp, temp2;
    register int nbits;
    register int k, r, i;

    /* Encode the DC coefficient difference per section F.1.2.1 */

    temp = temp2 = block[0] - last_dc_val;

    if ( temp < 0 ) {
        temp = -temp;   /* temp is abs value of input */
        /* For a negative input, want temp2 = bitwise complement of abs(input) */
        /* This code assumes we are on a two's complement machine */
        temp2--;
    }

    /* Find the number of bits needed for the magnitude of the coefficient */
    nbits = 0;
    while ( temp ) {
        nbits++;
        temp >>= 1;
    }

    /* Emit the Huffman-coded symbol for the number of bits */
    if ( !emit_bits( state, dctbl->ehufco[nbits], dctbl->ehufsi[nbits] ) ) {
        return FALSE;
    }

    /* Emit that number of bits of the value, if positive, */
    /* or the complement of its magnitude, if negative. */
    if ( nbits ) {      /* emit_bits rejects calls with size 0 */
        if ( !emit_bits( state, (unsigned int) temp2, nbits ) ) {
            return FALSE;
        }
    }

    /* Encode the AC coefficients per section F.1.2.2 */

    r = 0;          /* r = run length of zeros */

    for ( k = 1; k < DCTSIZE2; k++ ) {
        if ( ( temp = block[jpeg_natural_order[k]] ) == 0 ) {
            r++;
        } else {
            /* if run length > 15, must emit special run-length-16 codes (0xF0) */
            while ( r > 15 ) {
                if ( !emit_bits( state, actbl->ehufco[0xF0], actbl->ehufsi[0xF0] ) ) {
                    return FALSE;
                }
                r -= 16;
            }

            temp2 = temp;
            if ( temp < 0 ) {
                temp = -temp;/* temp is abs value of input */
                /* This code assumes we are on a two's complement machine */
                temp2--;
            }

            /* Find the number of bits needed for the magnitude of the coefficient */
            nbits = 1;  /* there must be at least one 1 bit */
            while ( ( temp >>= 1 ) ) {
                nbits++;
            }

            /* Emit Huffman symbol for run length / number of bits */
            i = ( r << 4 ) + nbits;
            if ( !emit_bits( state, actbl->ehufco[i], actbl->ehufsi[i] ) ) {
                return FALSE;
            }

            /* Emit that number of bits of the value, if positive, */
            /* or the complement of its magnitude, if negative. */
            if ( !emit_bits( state, (unsigned int) temp2, nbits ) ) {
                return FALSE;
            }

            r = 0;
        }
    }

    /* If the last coef(s) were zero, emit an end-of-block code */
    if ( r > 0 ) {
        if ( !emit_bits( state, actbl->ehufco[0], actbl->ehufsi[0] ) ) {
            return FALSE;
        }
    }

    return TRUE;
}


/*
 * Emit a restart marker & resynchronize predictions.
 */

LOCAL boolean
emit_restart( working_state * state, int restart_num ) {
    int ci;

    if ( !flush_bits( state ) ) {
        return FALSE;
    }

    emit_byte( state, 0xFF, return FALSE );
    emit_byte( state, JPEG_RST0 + restart_num, return FALSE );

    /* Re-initialize DC predictions to 0 */
    for ( ci = 0; ci < state->cinfo->comps_in_scan; ci++ ) {
        state->cur.last_dc_val[ci] = 0;
    }

    /* The restart counter is not updated until we successfully write the MCU. */

    return TRUE;
}


/*
 * Encode and output one MCU's worth of Huffman-compressed coefficients.
 */

METHODDEF boolean
encode_mcu_huff( j_compress_ptr cinfo, JBLOCKROW * MCU_data ) {
    huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
    working_state state;
    int blkn, ci;
    jpeg_component_info * compptr;

    /* Load up working state */
    state.next_output_byte = cinfo->dest->next_output_byte;
    state.free_in_buffer = cinfo->dest->free_in_buffer;
    ASSIGN_STATE( state.cur, entropy->saved );
    state.cinfo = cinfo;

    /* Emit restart marker if needed */
    if ( cinfo->restart_interval ) {
        if ( entropy->restarts_to_go == 0 ) {
            if ( !emit_restart( &state, entropy->next_restart_num ) ) {
                return FALSE;
            }
        }
    }

    /* Encode the MCU data blocks */
    for ( blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++ ) {
        ci = cinfo->MCU_membership[blkn];
        compptr = cinfo->cur_comp_info[ci];
        if ( !encode_one_block( &state,
                                MCU_data[blkn][0], state.cur.last_dc_val[ci],
                                entropy->dc_derived_tbls[compptr->dc_tbl_no],
                                entropy->ac_derived_tbls[compptr->ac_tbl_no] ) ) {
            return FALSE;
        }
        /* Update last_dc_val */
        state.cur.last_dc_val[ci] = MCU_data[blkn][0][0];
    }

    /* Completed MCU, so update state */
    cinfo->dest->next_output_byte = state.next_output_byte;
    cinfo->dest->free_in_buffer = state.free_in_buffer;
    ASSIGN_STATE( entropy->saved, state.cur );

    /* Update restart-interval state too */
    if ( cinfo->restart_interval ) {
        if ( entropy->restarts_to_go == 0 ) {
            entropy->restarts_to_go = cinfo->restart_interval;
            entropy->next_restart_num++;
            entropy->next_restart_num &= 7;
        }
        entropy->restarts_to_go--;
    }

    return TRUE;
}


/*
 * Finish up at the end of a Huffman-compressed scan.
 */

METHODDEF void
finish_pass_huff( j_compress_ptr cinfo ) {
    huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
    working_state state;

    /* Load up working state ... flush_bits needs it */
    state.next_output_byte = cinfo->dest->next_output_byte;
    state.free_in_buffer = cinfo->dest->free_in_buffer;
    ASSIGN_STATE( state.cur, entropy->saved );
    state.cinfo = cinfo;

    /* Flush out the last data */
    if ( !flush_bits( &state ) ) {
        ERREXIT( cinfo, JERR_CANT_SUSPEND );
    }

    /* Update state */
    cinfo->dest->next_output_byte = state.next_output_byte;
    cinfo->dest->free_in_buffer = state.free_in_buffer;
    ASSIGN_STATE( entropy->saved, state.cur );
}


/*
 * Huffman coding optimization.
 *
 * This actually is optimization, in the sense that we find the best possible
 * Huffman table(s) for the given data.  We first scan the supplied data and
 * count the number of uses of each symbol that is to be Huffman-coded.
 * (This process must agree with the code above.)  Then we build an
 * optimal Huffman coding tree for the observed counts.
 *
 * The JPEG standard requires Huffman codes to be no more than 16 bits long.
 * If some symbols have a very small but nonzero probability, the Huffman tree
 * must be adjusted to meet the code length restriction.  We currently use
 * the adjustment method suggested in the JPEG spec.  This method is *not*
 * optimal; it may not choose the best possible limited-length code.  But
 * since the symbols involved are infrequently used, it's not clear that
 * going to extra trouble is worthwhile.
 */

#ifdef ENTROPY_OPT_SUPPORTED


/* Process a single block's worth of coefficients */

LOCAL void
htest_one_block( JCOEFPTR block, int last_dc_val,
                 long dc_counts[], long ac_counts[] ) {
    register int temp;
    register int nbits;
    register int k, r;

    /* Encode the DC coefficient difference per section F.1.2.1 */

    temp = block[0] - last_dc_val;
    if ( temp < 0 ) {
        temp = -temp;
    }

    /* Find the number of bits needed for the magnitude of the coefficient */
    nbits = 0;
    while ( temp ) {
        nbits++;
        temp >>= 1;
    }

    /* Count the Huffman symbol for the number of bits */
    dc_counts[nbits]++;

    /* Encode the AC coefficients per section F.1.2.2 */

    r = 0;          /* r = run length of zeros */

    for ( k = 1; k < DCTSIZE2; k++ ) {
        if ( ( temp = block[jpeg_natural_order[k]] ) == 0 ) {
            r++;
        } else {
            /* if run length > 15, must emit special run-length-16 codes (0xF0) */
            while ( r > 15 ) {
                ac_counts[0xF0]++;
                r -= 16;
            }

            /* Find the number of bits needed for the magnitude of the coefficient */
            if ( temp < 0 ) {
                temp = -temp;
            }

            /* Find the number of bits needed for the magnitude of the coefficient */
            nbits = 1;  /* there must be at least one 1 bit */
            while ( ( temp >>= 1 ) ) {
                nbits++;
            }

            /* Count Huffman symbol for run length / number of bits */
            ac_counts[( r << 4 ) + nbits]++;

            r = 0;
        }
    }

    /* If the last coef(s) were zero, emit an end-of-block code */
    if ( r > 0 ) {
        ac_counts[0]++;
    }
}


/*
 * Trial-encode one MCU's worth of Huffman-compressed coefficients.
 * No data is actually output, so no suspension return is possible.
 */

METHODDEF boolean
encode_mcu_gather( j_compress_ptr cinfo, JBLOCKROW * MCU_data ) {
    huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
    int blkn, ci;
    jpeg_component_info * compptr;

    /* Take care of restart intervals if needed */
    if ( cinfo->restart_interval ) {
        if ( entropy->restarts_to_go == 0 ) {
            /* Re-initialize DC predictions to 0 */
            for ( ci = 0; ci < cinfo->comps_in_scan; ci++ ) {
                entropy->saved.last_dc_val[ci] = 0;
            }
            /* Update restart state */
            entropy->restarts_to_go = cinfo->restart_interval;
        }
        entropy->restarts_to_go--;
    }

    for ( blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++ ) {
        ci = cinfo->MCU_membership[blkn];
        compptr = cinfo->cur_comp_info[ci];
        htest_one_block( MCU_data[blkn][0], entropy->saved.last_dc_val[ci],
                         entropy->dc_count_ptrs[compptr->dc_tbl_no],
                         entropy->ac_count_ptrs[compptr->ac_tbl_no] );
        entropy->saved.last_dc_val[ci] = MCU_data[blkn][0][0];
    }

    return TRUE;
}


/*
 * Generate the optimal coding for the given counts, fill htbl.
 * Note this is also used by jcphuff.c.
 */

GLOBAL void
jpeg_gen_optimal_table( j_compress_ptr cinfo, JHUFF_TBL * htbl, long freq[] ) {
#define MAX_CLEN 32     /* assumed maximum initial code length */
    UINT8 bits[MAX_CLEN + 1];/* bits[k] = # of symbols with code length k */
    int codesize[257];      /* codesize[k] = code length of symbol k */
    int others[257];    /* next symbol in current branch of tree */
    int c1, c2;
    int p, i, j;
    long v;

    /* This algorithm is explained in section K.2 of the JPEG standard */

    MEMZERO( bits, SIZEOF( bits ) );
    MEMZERO( codesize, SIZEOF( codesize ) );
    for ( i = 0; i < 257; i++ ) {
        others[i] = -1;
    }                   /* init links to empty */

    freq[256] = 1;      /* make sure there is a nonzero count */
    /* Including the pseudo-symbol 256 in the Huffman procedure guarantees
     * that no real symbol is given code-value of all ones, because 256
     * will be placed in the largest codeword category.
     */

    /* Huffman's basic algorithm to assign optimal code lengths to symbols */

    for (;; ) {
        /* Find the smallest nonzero frequency, set c1 = its symbol */
        /* In case of ties, take the larger symbol number */
        c1 = -1;
        v = 1000000000L;
        for ( i = 0; i <= 256; i++ ) {
            if ( ( freq[i] ) && ( freq[i] <= v ) ) {
                v = freq[i];
                c1 = i;
            }
        }

        /* Find the next smallest nonzero frequency, set c2 = its symbol */
        /* In case of ties, take the larger symbol number */
        c2 = -1;
        v = 1000000000L;
        for ( i = 0; i <= 256; i++ ) {
            if ( ( freq[i] ) && ( freq[i] <= v ) && ( i != c1 ) ) {
                v = freq[i];
                c2 = i;
            }
        }

        /* Done if we've merged everything into one frequency */
        if ( c2 < 0 ) {
            break;
        }

        /* Else merge the two counts/trees */
        freq[c1] += freq[c2];
        freq[c2] = 0;

        /* Increment the codesize of everything in c1's tree branch */
        codesize[c1]++;
        while ( others[c1] >= 0 ) {
            c1 = others[c1];
            codesize[c1]++;
        }

        others[c1] = c2;    /* chain c2 onto c1's tree branch */

        /* Increment the codesize of everything in c2's tree branch */
        codesize[c2]++;
        while ( others[c2] >= 0 ) {
            c2 = others[c2];
            codesize[c2]++;
        }
    }

    /* Now count the number of symbols of each code length */
    for ( i = 0; i <= 256; i++ ) {
        if ( codesize[i] ) {
            /* The JPEG standard seems to think that this can't happen, */
            /* but I'm paranoid... */
            if ( codesize[i] > MAX_CLEN ) {
                ERREXIT( cinfo, JERR_HUFF_CLEN_OVERFLOW );
            }

            bits[codesize[i]]++;
        }
    }

    /* JPEG doesn't allow symbols with code lengths over 16 bits, so if the pure
     * Huffman procedure assigned any such lengths, we must adjust the coding.
     * Here is what the JPEG spec says about how this next bit works:
     * Since symbols are paired for the longest Huffman code, the symbols are
     * removed from this length category two at a time.  The prefix for the pair
     * (which is one bit shorter) is allocated to one of the pair; then,
     * skipping the BITS entry for that prefix length, a code word from the next
     * shortest nonzero BITS entry is converted into a prefix for two code words
     * one bit longer.
     */

    for ( i = MAX_CLEN; i > 16; i-- ) {
        while ( bits[i] > 0 ) {
            j = i - 2;  /* find length of new prefix to be used */
            while ( bits[j] == 0 ) {
                j--;
            }

            bits[i] -= 2;/* remove two symbols */
            bits[i - 1]++;/* one goes in this length */
            bits[j + 1] += 2;/* two new symbols in this length */
            bits[j]--;  /* symbol of this length is now a prefix */
        }
    }

    /* Remove the count for the pseudo-symbol 256 from the largest codelength */
    while ( bits[i] == 0 ) {/* find largest codelength still in use */
        i--;
    }
    bits[i]--;

    /* Return final symbol counts (only for lengths 0..16) */
    MEMCOPY( htbl->bits, bits, SIZEOF( htbl->bits ) );

    /* Return a list of the symbols sorted by code length */
    /* It's not real clear to me why we don't need to consider the codelength
     * changes made above, but the JPEG spec seems to think this works.
     */
    p = 0;
    for ( i = 1; i <= MAX_CLEN; i++ ) {
        for ( j = 0; j <= 255; j++ ) {
            if ( codesize[j] == i ) {
                htbl->huffval[p] = (UINT8) j;
                p++;
            }
        }
    }

    /* Set sent_table FALSE so updated table will be written to JPEG file. */
    htbl->sent_table = FALSE;
}


/*
 * Finish up a statistics-gathering pass and create the new Huffman tables.
 */

METHODDEF void
finish_pass_gather( j_compress_ptr cinfo ) {
    huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
    int ci, dctbl, actbl;
    jpeg_component_info * compptr;
    JHUFF_TBL ** htblptr;
    boolean did_dc[NUM_HUFF_TBLS];
    boolean did_ac[NUM_HUFF_TBLS];

    /* It's important not to apply jpeg_gen_optimal_table more than once
     * per table, because it clobbers the input frequency counts!
     */
    MEMZERO( did_dc, SIZEOF( did_dc ) );
    MEMZERO( did_ac, SIZEOF( did_ac ) );

    for ( ci = 0; ci < cinfo->comps_in_scan; ci++ ) {
        compptr = cinfo->cur_comp_info[ci];
        dctbl = compptr->dc_tbl_no;
        actbl = compptr->ac_tbl_no;
        if ( !did_dc[dctbl] ) {
            htblptr = &cinfo->dc_huff_tbl_ptrs[dctbl];
            if ( *htblptr == NULL ) {
                *htblptr = jpeg_alloc_huff_table( (j_common_ptr) cinfo );
            }
            jpeg_gen_optimal_table( cinfo, *htblptr, entropy->dc_count_ptrs[dctbl] );
            did_dc[dctbl] = TRUE;
        }
        if ( !did_ac[actbl] ) {
            htblptr = &cinfo->ac_huff_tbl_ptrs[actbl];
            if ( *htblptr == NULL ) {
                *htblptr = jpeg_alloc_huff_table( (j_common_ptr) cinfo );
            }
            jpeg_gen_optimal_table( cinfo, *htblptr, entropy->ac_count_ptrs[actbl] );
            did_ac[actbl] = TRUE;
        }
    }
}


#endif /* ENTROPY_OPT_SUPPORTED */


/*
 * Module initialization routine for Huffman entropy encoding.
 */

GLOBAL void
jinit_huff_encoder( j_compress_ptr cinfo ) {
    huff_entropy_ptr entropy;
    int i;

    entropy = (huff_entropy_ptr)
              ( *cinfo->mem->alloc_small )( (j_common_ptr) cinfo, JPOOL_IMAGE,
                                           SIZEOF( huff_entropy_encoder ) );
    cinfo->entropy = (struct jpeg_entropy_encoder *) entropy;
    entropy->pub.start_pass = start_pass_huff;

    /* Mark tables unallocated */
    for ( i = 0; i < NUM_HUFF_TBLS; i++ ) {
        entropy->dc_derived_tbls[i] = entropy->ac_derived_tbls[i] = NULL;
#ifdef ENTROPY_OPT_SUPPORTED
        entropy->dc_count_ptrs[i] = entropy->ac_count_ptrs[i] = NULL;
#endif
    }
}
